Manufacturing method for wide-range fine-grained magnesium alloy thin-sheet material

ABSTRACT

A manufacturing method for wide-range fine-grained magnesium alloy thin-sheet material is disclosed. The method includes an extrusion process and a rolling process. By the plastic deformation feature of the two processes, the wide-range fine-grained magnesium alloy thin-sheet material that satisfies the requirement of cases of 3C products with thickness of less than 1 mm is produced. Thus the method overcomes shortcomings of a conventional method that produces the material by a plurality passes of processes. Therefore, the manufacturing cost is reduced and the method is able to be applied to various industries.

BACKGROUND OF THE INVENTION

The present invention relates to a manufacturing method for sheet material, especially to a manufacturing method for wide-range fine-grained magnesium alloy thin-sheet material.

In AZ31 and AZ61 magnesium alloy sheet industry, there are still several difficulties to be overcome. Firstly, in a first processing process of the material from slab to rolling plate, due to coarse grain and segregation of casting slab, it's not easy to perform metal plastic rolling processes and thickness reduction ratio of rolling is restricted. Once rolling temperature is increased for improving rollingability, magnesium alloy material is easy to be oxidized so that the surface finish is influenced. Moreover, other problems such as low heat content of magnesium alloy, fast heat dissipation caused by high surface area/volume ratio of the sheet material, and difficulties in uniform temperature of slab/plate are also raised.

Due to hexagonal close packed crystal structure and high work hardening rate of magnesium, the slab needs to be rolled at least over ten passes and gradually eliminating cast structure as well as reducing thickness. Moreover, after the grains being refined, the rolling reduction is increased and surface oxides need to be removed (including turning, grinding and acid cleaning). Then the material is put into a heating furnace and is heated into the rolling temperature for performing next rolling process. There are too many passes of processes for producing magnesium alloy rolled sheet material so that cost of production is quite high. This restricts the applications of magnesium alloy sheet material in various industries.

Furthermore, under the restrictions of hexagonal close packing (hcp) crystal structure of the magnesium alloy, and difficulties in plastic deformation of the magnesium alloy at room temperature, the formability of these rolling sheet is not as good as other cold working sheet material (such as aluminum sheet). Thus there are limits on forming of complicated products.

Forming techniques such as press forging and press forming applied to processing of magnesium foil in industries have got some breakthrough.

For example, Sony Japan Co., Hitachi Metals Co., and Tokyo seitan Co. have develop a new forming technique of thin-walled material. In 2000, Panasonic MD also develops similar technique. These press forging and press forming techniques have advantages of auto-production, high yield rate and superior quality although manufacturing cost of slab rolling processes is still high. In future, once manufacturing cost of magnesium alloy sheet material can be reduced and forming properties of magnesium alloy sheet material can be improved for being applied with press forging and press forming techniques, main stream of magnesium alloy cases produced by magnesium die-casting in 3C (Consumer, Computer and Communication) industries will be changed.

SUMMARY OF THE INVENTION

Therefore it is a primary object of the present invention to provide a manufacturing method for wide-range fine-grained magnesium alloy thin-sheet material that combines an extrusion process with a rolling process. By the plastic deformation feature of the two processes, the wide-range fine-grained magnesium alloy thin-sheet material that satisfies the requirement of cases of 3C products with thickness of less than 1 mm is produced.

It is another object of the present invention to provide a manufacturing method for wide-range fine-grained magnesium alloy thin-sheet material that reduces passes of processes of a conventional method so as to reduce the manufacturing cost. Thus the method is able to be applied to various industries.

In order to achieve above objects, a manufacturing method for wide-range fine-grained magnesium alloy thin-sheet material according to the present invention includes a plurality of steps. Firstly, take a magnesium alloy billet. Then, perform an extrusion process. The magnesium alloy billet is extruded through a U-shaped hole of an extrusion die so as to get an U-shaped extruded material. Next, perform a pressing and leveling process. The U-shaped extruded material is pressed and leveled to a wide-range sheet material. Then the wide-range sheet material is rolled into a wide-range thin-sheet material that is further annealed to get a wide-range fine-grained magnesium alloy thin-sheet material.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein

FIG. 1 is a flow chart of an embodiment according to the present invention;

FIG. 2 is a cross-sectional view of an extrusion die of an embodiment according to the present invention;

FIG. 3 is a schematic drawing showing a U-shaped extruded material of an embodiment according to the present invention;

FIG. 4 is a schematic drawing showing a pressing and leveling process of an embodiment according to the present invention;

FIG. 5A is a metallographic image of wide-range thin-sheet material of an embodiment according to the present invention;

FIG. 5B is a metallographic image of wide-range fine-grained magnesium alloy thin-sheet material of an embodiment according to the present invention;

FIG. 5C a list of physical and mechanical properties of the wide-range thin-sheet material as well as the wide-range fine-grained magnesium alloy thin-sheet material of an embodiment according to the present invention;

FIG. 6A is a metallographic image of wide-range thin-sheet material of another embodiment according to the present invention;

FIG. 6B is a metallographic image of wide-range fine-grained magnesium alloy thin-sheet material of another embodiment according to the present invention;

FIG. 6C a list of physical and mechanical properties of the wide-range thin-sheet material as well as the wide-range fine-grained magnesium alloy thin-sheet material of another embodiment according to the present invention;

FIG. 7A is a metallographic image of wide-range thin-sheet material of a further embodiment according to the present invention;

FIG. 7B is a metallographic image of wide-range fine-grained magnesium alloy thin-sheet material of a further embodiment according to the present invention;

FIG. 7C a list of physical and mechanical properties of the wide-range thin-sheet material as well as the wide-range fine-grained magnesium alloy thin-sheet material of a further embodiment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to manufacture a wide-range magnesium alloy thin-sheet material that satisfies requirements of cases of 3C products, a manufacturing method for wide-range fine-grained magnesium alloy thin-sheet material includes following steps:

In the beginning, run the step S10, take a magnesium alloy billet. Then, perform an extrusion process, as shown in the step S12. The billet is extruded through a U-shaped hole 101 of an extrusion die 10 so as to get an U-shaped extruded material 2. Refer to FIG. 2, ratio of diameter of the U-shaped hole 101 to the extrusion die 10 is smaller than 0.7. The thickness of the U-shaped hole 101 ranges from 1.5 mm to 5 mm while 2.0 mm is preferred. The U-shaped extruded material 2 produced by the extrusion process is shown in FIG. 3.

Refer to the step S14, perform a pressing and leveling process. The U-shaped extruded material 2 is pressed and leveled to a wide-range sheet material. Before performing the pressing and leveling process, cut the U-shaped extruded material 2 into required length according to users' needs. The cut U-shaped extruded material 2 is set between two iron plates 3 for the pressing and leveling process. The thickness of each iron plate 3 is 25 mm and a working temperature of the process is between 250 degrees Celsius and 400 degrees Celsius while 350 degrees Celsius is preferable. By applying pressure to the two iron plates 3, the U-shaped extruded material 2 is leveled into wide-range sheet material, as shown in FIG. 4. Thus the extrusion limit of 0.7 D of width of the wide-range sheet material is broken through. Moreover, along with change of the vertex angle of the U-shaped hole, the width of the wide-range sheet material also changes between 0.7 and 2.1 times of the diameter of the magnesium alloy billet. Take the magnesium alloy billet with diameter of 203 mm as an example, the width of the extruded wide-range sheet material ranges from 142 mm to 425 mm while the optimum width of the wide-range sheet material is 1.3 times of the diameter of the magnesium alloy billet.

There is a limit value of the minimum thickness of the wide-range sheet material. When the thickness of the material is under this limit value, difference of flow rate of metal plastic flow between the edges and the center of the material is too large that results in tearing, bending, and uneven thickness. Thus the defective rate increases and the manufacturing cost also rises. Therefore the wide-range sheet material treated through the extrusion, pressing and leveling processes still doesn't satisfy the requirement of cases of 3C products with thickness of less than 1 mm. Due to the hexagonal close packing (hcp) crystal structure of the magnesium alloy, through the extrusion, pressing and leveling processes, the wide-range sheet material is affected by tensile stress and compressive stress so that the metallographic microstructure is deformation twinning-uneven grain size and distribution as well as obvious anisotropy of the material, even with a long wide twin band. These all have negative effects on plastic formability of the thin-sheet material. Therefore, in order to produce sheet material with required thickness, refining magnesium alloy grains evenly and eliminate anisotropy of the material, a specific warm rolling process is introduced.

After being treated with the pressing and leveling process, the wide-range sheet material with thickness between 1.5 mm and 5 mm is got. Take the step S16, roll the wide-range sheet material into a wide-range thin-sheet material. The material is rolled by a rolling machine disposed with a pair of roller. The roller is able to be heated to 200 degrees Celsius and the temperature of the roller is controlled between 100 degrees Celsius and 200 degrees Celsius while the optimum temperature is 120 degrees Celsius. The working speed of the roller ranges from 1 m/min to 4 m/min while 2.0 m/min is preferred. Owing to poor rollingability of the magnesium alloy at the room temperature, the wide-range sheet material needs to be heated to the temperature between 250 degrees Celsius and 400 degrees Celsius while 300 degrees Celsius is preferably. Then perform a pass of warm rolling process with various reduction ratio ranging from 20% to 70%. In accordance with the reduction rate, the wide-range thin-sheet material with required thickness is obtained. On the other hand, by control of conditions of warm rolling during the warm rolling and the thinning processes, the strain-deformed grains of the magnesium alloy release strain energy through dynamic recrystallization and become to even and fine equiaxed grains. The magnesium alloy grains are refined to an average size from 10 μm to even 5 μm ultra-fined grains. Thus the magnesium alloy sheet material manufactured by the present invention also has excellent superplasticity. As to the anisotropy of the extruded sheet material, it's gradually disappeared due to re-formation of even and fine equiaxed grains.

At last, take the step S18, anneal the wide-range thin-sheet material to get a wide-range fine-grained magnesium alloy thin-sheet material. The annealing temperature ranges from 250 degrees Celsius to 400 degrees Celsius while 300 degrees Celsius is optimum temperature. The annealing time is between 1 hour to 3 hours while one hour is preferably. In order to improve plastic formability of the wide-range fine-grained magnesium alloy thin-sheet material, the grain size should be controlled effectively and the mechanical properties of the material should be optimized.

In order to demonstrate the feature of evenness in longitudinal and crosswise directions of the wide-range fine-grained magnesium alloy thin-sheet material produced by the present invention, an anisotropic index that represents anisotropy of mechanical properties of sheet material is introduced. The tensile properties, grain size and the anisotropic index are used as references for comparing the wide-range fine-grained magnesium alloy thin-sheet material produced by the present invention with the commercial sheet material. The anisotropic index is calculated by the following equation:

anisotropic index=|(longitudinal properties−crosswise properties)/(longitudinal properties+crosswise properties)

wherein the longitudinal properties and crosswise properties include values of tensile strength, yield strength and elongation rate obtained through tensile tests at room temperature. Due to calculation of the absolute value, the value is no less than zero. The larger the anisotropic index is, the higher the anisotropy of mechanical properties of the sheet material in longitudinal and crosswise directions is. The higher anisotropy represents more difficulties in following plastic formability. Moreover, whether the grains are refined and equiaxed is also an important indicator for evaluating goodness of the material structure and anisotropy of the sheet material.

Refer from FIG. 5A to FIG. 5C, these respectively are a metallographic image of the wide-range thin-sheet material, a metallographic image of the wide-range fine-grained magnesium alloy thin-sheet material, and a list of their physical and mechanical properties. As shown in figure, take commercial AZ31 magnesium alloy billet with diameter of 203 mm being treated with wide-ranged extrusion process, and the pressing and leveling process so as to produce wide-range sheet material with the length of 310 mm, the width of 260 mm and the thickness of 3 mm. Next run the warm rolling process. The temperature of the roller of the rolling machine is controlled at 200 degrees Celsius and the speed thereof is 1 m/min while the wide-range sheet material is heated to 400 degrees Celsius. A pass of the warm rolling process with the 20% reduction ratio is run to produce the wide-range thin-sheet material with thickness of 2.4 mm. Then the wide-range thin-sheet material is annealed and the annealing temperature is controlled at 250 degrees Celsius and the annealing time is one hour so as to get the wide-range fine-grained magnesium alloy thin-sheet material. Then observe metallographic structure of the wide-range thin-sheet material through the warm-rolling process and the wide-range thin-sheet material treated through the warm-rolling process and the annealing process. Then take samples of both of them for performing mechanical tensile tests in longitudinal and crosswise directions.

Even without being treated with the annealing process, the wide-range fine-grained magnesium alloy thin-sheet material manufactured by the present invention has the even and fine equiaxed structure with an average grain size of 7.31 μm by means of dynamic recrystallization. Once the wide-range fine-grained thin-sheet material is further annealed so as to make grain size and the physical properties more optimize. Thus not only the strength and the elongation rate of the material is far more better than general magnesium alloy sheet material but the average of the anisotropy index is 0.4 to 0.6 times of the general magnesium alloy sheet material while its microstructure still keeps the even and fine equiaxed grain structure. These features have enhanced the plastic formability of the wide-range fine-grained magnesium alloy thin-sheet material dramatically. Therefore, the wide-range fine-grained magnesium alloy thin-sheet material manufactured by the present method has ideal microstructure and excellent physical properties. Moreover, it also has industrial applications due to fewer passes of the plastic forming process.

Refer from FIG. 6A to FIG. 6C, these respectively are a metallographic image of the wide-range thin-sheet material, a metallographic image of the wide-range fine-grained magnesium alloy thin-sheet material, and a list of their physical and mechanical properties of another embodiment according to the present invention. As shown in the figure, take commercial AZ31 magnesium alloy billet with diameter of 203 mm being treated with the wide-ranged extrusion process, and the pressing and leveling process so as to produce wide-range sheet material with the length of 310 mm, the width of 260 mm and the thickness of 1.5 mm. Next run the warm rolling process. The temperature of the roller of the rolling machine is controlled at 120 degrees Celsius and the speed thereof is 3 m/min while the wide-range sheet material is heated to 300 degrees Celsius. A pass of the warm rolling process with the 50% reduction ratio is run to produce the wide-range thin-sheet material with thickness of 0.75 mm. Then the wide-range thin-sheet material is annealed and the annealing temperature is controlled at 350 degrees Celsius and the annealing time is one hour so as to get the wide-range fine-grained magnesium alloy thin-sheet material. Then observe metallographic structure of the wide-range thin-sheet material through the warm-rolling process and the wide-range thin-sheet material through the warm-rolling process and the annealing process. Then take samples of both of the material for performing mechanical tensile tests in longitudinal and crosswise directions.

While performing the warm rolling process, heat the wide-range sheet material to 300 degrees Celsius so that the produced wide-range fine-grained magnesium alloy thin-sheet material has the even and fine equiaxed structure with an average grain size of 3.8 μm by means of dynamic recrystallization. Once the wide-range thin-sheet material is further annealed so as to make the microstructure more optimize. Thus not only the elongation rate of the material will not be sacrificed but the average of the anisotropic index is 0.017 that is quite ideal and the microstructure becomes the even and fine equiaxed grain structure. These features have enhanced the plastic formability of the wide-range fine-grained magnesium alloy thin-sheet material dramatically. Therefore, the wide-range fine-grained magnesium alloy thin-sheet material manufactured by the present method has ideal microstructure and excellent physical properties. Moreover, it also has industrial applications due to fewer passes of the plastic forming process.

Refer from FIG. 7A to FIG. 7C, these respectively are a metallographic image of the wide-range thin-sheet material, a metallographic image of the wide-range fine-grained magnesium alloy thin-sheet material, and a list of their physical and mechanical properties of a further embodiment according to the present invention. As shown in figures, take commercial AZ61 magnesium alloy billet with diameter of 203 mm being treated with the wide-ranged extrusion process, and the pressing and leveling process so as to produce wide-range sheet material with the length of 310 mm, the width of 260 mm and the thickness of 2 mm. Next run the warm rolling process. The temperature of the roller of the rolling machine is controlled at 150 degrees Celsius and the speed thereof is 2 m/min while the wide-range sheet material is heated to 350 degrees Celsius. A pass of the warm rolling process with the 40% reduction ratio is run to produce the wide-range thin-sheet material with thickness of 1.2 mm. Finally, the wide-range thin-sheet material is annealed and the annealing temperature is controlled at 300 degrees Celsius and the annealing time is one hour so as to get the wide-range fine-grained magnesium alloy thin-sheet material. Then observe metallographic structure of the wide-range thin-sheet material treated through the warm-rolling process and the wide-range thin-sheet material through the warm-rolling process and the annealing process. Then take samples of both of the material for performing mechanical tensile tests in longitudinal and crosswise directions.

Due to high amount of aluminum, the AZ61 magnesium alloy billet has higher deformation resistance. After the warm rolling process, grains of the wide-range sheet material has large amount of strain energy and in non-equiaxed structure, as shown in FIG. 7A. However, through the annealing process, the grains turn to equiaxed structure with an average grain size of 5.1 μm, as shown in FIG. 7B. Beside high strength, after the annealing process, the average anisotropic index of the wide-range fine-grained magnesium alloy thin-sheet material is more ideal than general magnesium alloy sheet material. With evenly and fine microstructure and the average grain size of 5.1 μm, these are all beneficial to plastic formability of the material. Therefore, the wide-range fine-grained magnesium alloy thin-sheet material manufactured by the present method obviously has ideal microstructure and excellent physical properties. Moreover, it also has industrial applications due to fewer passes of the plastic forming process.

In summary, the present invention provides a manufacturing method for wide-range fine-grained magnesium alloy thin-sheet material that combines features of an extrusion process with a rolling process. Compared with conventional die-casting or semi-solid injection, the present invention not only has advantages of high yield rate, superior surface finish, simple post processes, good heat dissipation and high material usage rate but also prevents shortcomings of a plurality of passes of processes. Thus the manufacturing cost is dramatically reduced. Therefore, the method has industrial applications and has great potential in material thinning.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

1. A manufacturing method for wide-range fine-grained magnesium alloy thin-sheet material comprising the steps of: providing a magnesium alloy billet; extruding the magnesium alloy billet through a U-shaped die hole to get an U-shaped extruded material; pressing and leveling the U-shaped extruded material into a wide-range sheet material; rolling the wide-range sheet material into a wide-range thin-sheet material; and annealing the wide-range thin-sheet material to wide-range fine-grained magnesium alloy thin-sheet material.
 2. The method as claimed in claim 1, wherein thickness of the U-shaped die hole ranges from 1.5 mm to 5 mm.
 3. The method as claimed in claim 2, wherein optimum thickness of the U-shaped die hole is 2.0 mm.
 4. The method as claimed in claim 1, wherein width of the wide-range sheet material ranges from 0.7 to 2.1 times of the diameter of the magnesium alloy billet.
 5. The method as claimed in claim 4, wherein optimum width of the wide-range sheet material is 1.3 times of the diameter of the magnesium alloy billet.
 6. The method as claimed in claim 1, wherein working temperature of the step of pressurizing and leveling the U-shaped extruded material is between 250 degrees Celsius and 400 degrees Celsius.
 7. The method as claimed in claim 6, wherein optimum working temperature of the step of pressing and leveling the U-shaped extruded material is 350 degrees Celsius.
 8. The method as claimed in claim 1, wherein a roller is used to roll the wide-range sheet material in the step of rolling the wide-range sheet material.
 9. The method as claimed in claim 8, wherein temperature of the roller is between 100 degrees Celsius and 200 degrees Celsius.
 10. The method as claimed in claim 9, wherein optimum temperature of the roller is 120 degrees Celsius.
 11. The method as claimed in claim 8, wherein working speed of the roller ranges from 1 m/min to 4 m/min.
 12. The method as claimed in claim 8, wherein optimum working speed of the roller is 2.0 m/min.
 13. The method as claimed in claim 1, wherein the step of rolling the wide-range sheet material further comprising a step of: heating the wide-range sheet material.
 14. The method as claimed in claim 13, wherein temperature for heating the wide-range sheet material ranges from 250 degrees Celsius to 400 degrees Celsius.
 15. The method as claimed in claim 13, wherein optimum temperature for heating the wide-range sheet material is 300 degrees Celsius.
 16. The method as claimed in claim 1, wherein reduction ratio of the step of rolling the wide-range sheet material ranges from 20% to 70%.
 17. The method as claimed in claim 16, wherein optimum reduction ratio of the step of rolling the wide-range sheet material is 50%.
 18. The method as claimed in claim 1, wherein temperature for annealing the wide-range thin-sheet material ranges from 250 degrees Celsius to 400 degrees Celsius
 19. The method as claimed in claim 18, wherein optimum temperature for annealing the wide-range thin-sheet material is 300 degrees Celsius.
 20. The method as claimed in claim 1, wherein a duration of the step of annealing the wide-range thin-sheet material ranges from 1 hour to 3 hours.
 21. The method as claimed in claim 20, wherein optimum duration of the step of annealing the wide-range thin-sheet material is 1 hour. 